Spectral Absorption Monitoring ("SAM") systems operate on the principle that constituent molecular gases within a sample volume of gas have a number of known, narrow absorption lines in the infrared portion of the electromagnetic spectrum. These absorptions lines are associated with transitions from a ground state to a higher energy level of a molecule upon absorption of photons. This higher energy level corresponds to an excited vibrational or rotational state of the molecule. All molecular vapors have absorption lines within the infrared region. Broad band SAM systems are concerned with the determination of the types and quantities of constituent vapors in a sample of gas. Narrow band systems are designed to unambiguously monitor the concentration of a particular vapor. Sensitivity to ultralow concentration and real time measurement are key requirements of narrow band systems, such as the invention described herein.
By transmitting a band of optical energy having a narrow range of wavelengths in and around the peak of an absorption line of a particular constituent, the absorptivity or proportional reduction in transmitted power due to a molecular absorption line of that constituent can be measured to determine its concentration. When this is performed using a measurement cell containing a sample of air of fixed length at a known temperature and pressure, the concentration of the particular constituent believed to be present in the measurement cell can be determined even with very low concentrations. Current sensitive measurement systems have required that the source of spectral output be narrow in wavelength range compared to the half width of the chosen molecular absorption line.
Several constituents may absorb optical energy at the same wavelength. To resolve an ambiguity as to which constituent has what concentration, some absorption systems are chosen to be sensitive to the narrowness of the absorption line, i.e., the absorption signal is proportional to the rate of change of absorption with wavelength. Mathematically, this is expressed as: EQU S.sub.d .varies.dA(.lambda.)/d.lambda.
where S.sub.d is the electrical signal detected, A is the absorptivity of the constituent as a function of wavelength, and .lambda. is the wavelength. Thus, broad band absorption or scattering within the measurement cell will produce a greatly reduced or possibly no absorption signal because the rate of change of absorptivity with wavelength is negligible. In contrast, the wavelength of the peak absorption of a narrow line is a pretty good signature of a particular constituent. Of course, if a second absorption line of the same constituent vapor is monitored as well a virtually certain signature will be obtained.
Today, narrow band SAM systems tend to be constructed around coherent light sources, typically, single frequency semiconductor laser diodes. Current narrow band spectroscopic research typically utilizes II-VI (lead-salt) semiconductor diode lasers operating in the 3-10 micron spectral region. Such sources are cryogenically cooled and therefore are more expensive and more cumbersome than diode lasers constructed from III-V semiconductor materials such as InGaAsP/InP and GaAs/AlGaAs diodes which operate in the shorter red and near-infrared wavelengths from about 0.63 to 1.55 micron. Nevertheless, the lead-salt laser instruments constructed to date have routinely achieved pans-per-billion (ppb) detection levels of a number of important molecular species. See Near-Infrared Diode Lasers Monitor Molecular Species, Laser Focus World, November, 1992, p. 133. The ability to monitor species at ppb levels is of interest to manufacturers of high purity gases used in the semiconductor device fabrication industry where impurities such as H.sub.2 O are damaging and reduce the yield of operational circuits. The monitoring of narrow band absorption lines due to water vapor is a particularly useful application of the present invention. Also, air in a workplace or factory can be monitored to meet clean air requirements. Sensitivity to ultralow concentrations has applications in medical diagnostics and in process control.